I am plotting a lower triangular matrix of plots with groupplot, and I would like to add a title to the plots on the diagonal. I generate these plots by reading data using \foreach, and therefore need to access the title strings via either a file or a list stored in pgfplots.

The title file would have a math environment in them, like the code below. Is there a way to apply these values to the plot below? For this example the three diagonal plots would have the titles {$\alpha_1$, $\alpha_2$, $beta$}.

Thanks again! Oddly enough, you can't seem to use \mathrm{} in the titles, as it throws an error. For the time being I'll just use \text{}, but may look into why in the future if it bugs me.
– chaselyApr 16 '18 at 0:22

@chasely I confirm your observation, and was not able to cure this with a reasonable amount of \noexpands. Nor am I able to fix the lualatex issue. These might really important questions for real experts like Christian Feuersänger or Torbjørn T..
– user121799Apr 16 '18 at 1:09